In the related art, a thin-film transistor (TFT) in which amorphous silicon or polycrystalline silicon is used as a material has been widely used as a switching element for a liquid crystal display device, an organic EL display device, and the like. However, a CVD apparatus used for producing a transistor using the silicon materials is expensive and thus, producing of a large transistor integrated circuit leads to an increase in producing cost. In addition, the silicon material is formed into a film at high temperature, and therefore, due to a problem of heat resistance, cannot be applied to a next generation flexible display device in which a plastic substrate is supposed to be used. In order to solve this problem, an organic transistor in which an organic semiconductor material is used as a channel (semiconductor layer) instead of the silicon semiconductor material has been proposed.
Since it is possible to form a film at low temperature by using the organic semiconductor material as an ink, a large-scale production facility is not required. In addition, the organic semiconductor material can be applied to a plastic substrate with poor heat resistance and has been expected to be applied to flexible electronics.
Such an organic semiconductor material had a problem in that, since semiconductor properties (mobility) are poor as compared with the silicon semiconductor materials, a response speed of the transistor is slow and thereby it is difficult to put it into practical use; however, in recent years, materials, the mobility of which exceeds that of amorphous silicon, have been developed.
For example, PTL 1 discloses that a compound having a dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene skeleton exhibits mobility of 4.0 cm2/Vs in a vacuum evaporated film, PTLs 2 and 3 disclose that a dinaphtho[2,3-b:2′,3′-d]thiophene (hereinafter, abbreviated as dinaphthothiophene) derivative exhibits a high mobility of 11 cm2/Vs in a single crystal thin film formed by an edge casting method, NPL 1 discloses that 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene exhibits a high mobility of 30 cm2/Vs in a single crystal film prepared by a double ink jet method while its properties have large variation, PTL 4 discloses that a phenyl substituted naphthodichalcogen compound exhibits a mobility of 0.7 cm2/Vs, and PTLs 5 and 6 disclose that a compound exhibiting a highly ordered liquid crystal phase shows a mobility of 5.7 cm2/Vs in a thin film formed via a highly ordered liquid crystal phase. As such, the organic semiconductor materials exhibiting the semiconductor properties where the mobility thereof exceeds that of amorphous silicon (0.5 cm2/Vs) have been reported one after another.
Although the mobility of the organic semiconductors has been increased in this way, it has not yet been put to practical use. The reason is that the mobility as disclosed above is evaluated by using a semiconductor layer composed of a film prepared by a film-forming method which gives a homogeneous bulk such as a single crystal film prepared by an edge casting method or a double ink jet method, a thin film prepared by a vacuum deposition method and a thin film prepared by a spin coating method, and in the case of using the semiconductor layer made of a polycrystalline film which is prepared by dropping (drop-casting) an ink droplet and then drying it, that is, a practical printing method such as an ink jet method and a nozzle printing method, the properties are deteriorated.